The exhaust gas of a diesel engine or other internal combustion engine, for example, contains carbon monoxide (CO), unburned hydrocarbons (HC), particulate matter (PM), and also nitrogen oxides (NOx). As one method for removing the nitrogen oxides, it is known to arrange an exhaust purification system for reducing the NOx in an engine exhaust passage.
The system for reducing the NOx includes an NOx storage reduction catalyst which temporarily stores the NOx. The NOx storage reduction catalyst stores NO when the air-fuel ratio of the exhaust gas is large, that is, when the air-fuel ratio of the exhaust gas is lean. As opposed to this, when the air-fuel ratio of the exhaust gas is small, that is, when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio or rich, it releases the stored NOx and uses a reducing agent which is contained in the exhaust gas to reduce and remove the NOx.
The NOx storage reduction catalyst gradually accumulates NOx if use is continued. Further, when the exhaust gas which flows into the NOx storage reduction catalyst contains SOx, the SOx is stored. If the NOx storage reduction catalyst accumulates a lot of NOx or SOx, its ability to remove NOx from the exhaust gas falls. For this reason, the NOx storage reduction catalyst is regenerated by making it release the NOx or SOx. When regenerating it by making it release the NOx, the air-fuel ratio of the exhaust gas which flows into the NOx storage reduction catalyst is made the stoichiometric air-fuel ratio or rich. When regenerating it by making it release the SOx, the temperature of the NOx storage reduction catalyst is made the temperature enabling release of SOx or more, then the air-fuel ratio of the exhaust gas which flows into the NOx storage reduction catalyst is made the stoichiometric air-fuel ratio or rich.
Japanese Patent Publication (A) No. 2004-92431 discloses an exhaust gas purification system which has an exhaust passage having a first partial exhaust passage and a second partial exhaust passage and having the partial exchange passages converge into a common exhaust passage at the downstream side, NOx storage agents which are arranged in each partial exhaust passage, and an oxidation catalyst which is arranged in the common exhaust passage. In this exhaust gas purification system, when control for desorption of sulfur is performed at the NOx storage agent, the air-fuel ratio of the exhaust gas at each partial exhaust passage is controlled so that the air-fuel ratio of the exhaust gas which flows through the oxidation catalyst becomes slightly lean. In this exhaust gas purification system, it is disclosed that the control for desorption of sulfur of the NOx storing means can be efficiently performed and that H2S can be kept from being released to the outside.